1. Field of the Invention
The present invention relates to a femoral element for a total hip prosthesis and more particularly to the means for fixing it in the medullary canal of the femur.
2. History of the Related Art
It is known that, up to the present time, three processes exist for fixing hip-joint endoprostheses, namely either by fixing with the aid of a cement, by direct impaction, i.e. the engagement by controlled force of the femoral stem in the medullary canal without using cement, or by partial fixation and solely in the upper part of the medullary canal as described in U.S. Pat. No. 4,888,022.
For femoral prostheses fixed via a cement, two methods exist for implementation:
the acrylic cement is simply placed in the medullary canal and the prosthesis is impacted; PA0 a plug is placed in the medullary canal at about 1 cm beyond the end of the femoral stem, then cement is poured or injected in the canal and, finally, the stem is impacted in order to place the cement previously introduced under pressure. The excessive quantity of cement emerges via the upper end of the femur.
These cemented prostheses present certain drawbacks concerning the risks associated with the use of the cement and more particularly due to the rise in temperature and to a release of products which present a variable toxicity.
An immediate fixation is obviously obtained, but over the whole length of the stem without privileged zone, not taking into account the different zones of elasticity of the femur.
This zone of fixation changes the natural transmission of the stresses on the bone and, in the long run, modifies the osseous structures in the sense of deteriorating the fixation.
In the case of recovery, all the cement must be removed and, to that end, the plug of cement at the lower end of the prosthesis must be pierced, which is long and difficult work.
The femoral prostheses fixed without cement presents a surface appearance over all or part of the femoral stem so as to obtain an immediate stability in the medullary canal.
This method of fixation comprises certain drawbacks due to the tapering form of the stems of the prostheses, bringing about a very random contact in the metaphysal part of the femur and limited in the best cases to a few zones which cannot be foreseen. Moreover, the bone is subjected to stresses during fixation and to a considerable deterioration when the femoral prosthesis has to be withdrawn.
Finally, a micromobility may occur, bringing about a separation in the mean run.
The last process of fixation consists in a hip prosthesis of which the upper part is coated over the whole of its surface with an elastomer bladder. This bladder forms a retaining pouch upon introduction of a settable fluid. It inflates, applying against the inner walls of the medullary canal and more particularly in the diaphysal-metaphysal zone of the femur. Finally, the outer surface of the bladder comprises zones of rehabilitation of the bone which are disposed in a determined grid.
This method of fixation comprises certain drawbacks due to the operational stresses created by the introduction of a new element inside the medullary canal. Moreover, the elastomer bladder risks not being fluid-tight in its lower part under the effect of pressure during the introduction of the settable fluid. Finally, the positioning of this bladder on the body of the hip prosthesis considerably increases its cost price.